Camshaft drive mechanisms which incorporate variable valve timing (VVT) are known. The benefits in terms of engine performance are discussed in SAE Technical Paper Series 880386 entitled "A Review of Variable Engine Valve Timing" by C Gray which also discusses various variable valve timing mechanisms.
One type of VVT mechanism which gives a very large scope for improving engine performance varies the timing of both the opening and the closing of the valves by cyclic variation of the rotational speed of the cams during otherwise constant rotational speed of the engine crankshaft.
In applying this type of VVT mechanism to an engine with two or more cylinders there arises a problem of installation since the valve timing of each cylinder is at a different phase of the cyclic variation at any one time.
GB-A-1522405 illustrates the installation problem in a four cylinder in-line engine where a VVT mechanism is installed between No 2 cylinder and No 3 cylinder. Here the installation is made practicable by the engine being of a type commonly used in large motorcycles where the engine drive output is from a gear wheel on the crankshaft between cylinders No 2 and 3. A conventional four cylinder engine for cars or automobiles takes the engine drive output from one end of the crankshaft and it becomes impracticable to increase the length of the engine by installing a camshaft drive and VVT mechanism between cylinders Nos 2 and 3.
GB-A-1522405 illustrates a known VVT mechanism where the notionally constant rotational speed of a driving member produces a cyclic variation in the speed of a camshaft by moving the axis of rotation of the driving member relative to the axis of rotation of an output member on the camshaft. A drive mechanism transmits drive between the driving member and the output member. In GB-A-1522405 this drive mechanism comprises a drive peg on the output member which engages a slot in the driving member. U.S. Pat. No. 4,723,517 illustrates a VVT mechanism of the same general type but where the drive mechanism comprises a slide driven through a slot and tongue connection by the driving member and having rack teeth which engage gear teeth on the camshaft.
GB-A-1311562 overcomes the problem of providing a rotary drive to a driving member whose axis of rotation has to be moved by providing an input member which is coaxial with the output member and drives the driving member through a peg which engages a further slot in the driving member. The notionally constant rotational speed of the input member produces a cyclic variation in the rotational speed of the camshaft by moving the axis of rotation of the driving member. Although two methods of moving the driving member are shown, neither method approaches the optimum control of valve timing required to give good low speed performance and low exhaust emissions at idle or tickover combined with high power at higher engine speeds.